The studied samples were prepared by thermal evaporation of a thin Mo or W film on SiO2/Si or HfO2(2 nm)/SiO2/Si substrates. Next, the samples were transferred to a furnace and sulfurized in pure H2S(10 or 100 mbar) at 800 °C resulting in the formation of a large-area (in the range of cm2) poly-crystalline MoS2 or WS2 films with crystallites of 50 nm size . Cross-sectional TEM images reveal the characteristic layered structure. The Raman spectrum exhibits two distinct peaks corresponding to the in-plane (E2g) and the out-of-plane (A1g) vibrations in TMD layer, respectively. This observation reveals conversion of the metal film into 2H-MoS2 or 2H-WS2 hexagonal polytypes. By varying the thickness of the initially deposited metal, it appears possible to produce samples with 4 or 2 ML thick MoS2 and WS2 films without contamination of the sulfide/oxide interfaces. Electrical contacts were fabricated by evaporation of 100-nm thick Au or Al pads on top of the TMD films .
Fabrication of large area laterally uniform MoS2 and WS2 few-ML thin films enabled us to perform IPE experiments which address one of the fundamental issues concerning energy band alignment between different TMD materials. By observing electron photoemission from the valence band (VB) of MoS2 or WS2 into the conduction band (CB) of SiO2 underlayer, one can find the energy barriers. These correspond to the VB top energies of the two photo-emitting TMD materials referenced to the same energy level of the oxide CB bottom edge . The IPE quantum yield spectra reveal the same field-dependent energy onset of the IPE current in the energy range 3.5-4.0 eV for both MoS2 and WS2 as well as in their stacked heterojenctions. Since the same photocurrent spectra were also obtained when using Al as the contact pad material instead of Au, the TMD layer can be identified as the sole source of photoelectrons . This conclusion is independently affirmed by the observed significant decrease of the quantum yield if reducing the thickness of MoS2 electrode from 4 ML to 2 ML. The observed field dependence of the IPE spectra agrees well with the image-force barrier lowering at the TMD/SiO2 interface. The energy barrier of Fe =4.2±0.1 eV between the top of the MoS2 VB and the bottom of the SiO2 CB can be found by extrapolating this dependence to zero field. No significant difference between spectral thresholds from MoS2 and WS2 is found suggesting that the VBs of of these TMDs are nearly aligned. The same conclusion can be reached if analyzing the IPE yield spectra from MoS2/WS2 and WS2/MoS2 heterostructures. This result would suggest the validity of the “common anion rule” for sulfides of metals similarly to the earlier studied case of insulating metal oxides . Recent IPE experiments addressing the effects of TMD layer transfer on top of insulating substrate will also be discussed.
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